3.1 Spatial scale and nest predation (I)
Predation intensity in this study was extremely high; about 40 % of nests (N=108) were robbed after the first two days and 95 % after seven days. At landscape level, the total predation rate of nests was about equal between the landscape types, although there was a trend that predation risk in stands increased with increasing fragmentation (30.5 to 50.0 %). At stand size level, predation was clearly higher in large stands (64.8 %) than in small ones. This concentration also increased as a function of increased fragmentation of the surrounding matrix since the highest depredation rate was detected in large stands within the most fragmented landscape (94.4 %).
The unusually high nest predation pressure observed was probably caused by the decline of vole populations during the preceding winter in the study area. This decline can be well seen in the track densities of mammalian predators derived from the wildlife triangle censuses executed in Finnish Lapland (Helle et al. 1995).
The difference in the resource availability between forest stands and the surrounding matrix may affect the habitat use of predators. Many rodent
eating predators like foxes and small mustelids, may prefer to hunt in clear
cuts during vole peak years, because the density of voles there is higher than in closed forests (Hansson 1979, Lindstrom 1989). However, when vole populations crash in open areas, predators may penetrate to neighbouring forest stands to hunt alternative preys (Angelstam et al. 1984). In this situation a large forest patch could possible offer more food than a smaller one which may explain predation pattern observed in this study. This assumption was especially well supported by the observation that predation pressure was highest in the large stands within the fine-grained landscapes where the relative area of a open land was also highest.
3.2 Effect of the surrounding matrix (II)
15
In the forest-clear-cut dominated landscape the overall predation rate of artificial ground nests was not related to the distance from the forest-dear-cut edge or stand size. Instead, in the agricultural landscape nest predation was clearly increased in the edges in the proximity of fields. The edge nests situated very close to fields were preyed upon more often than the edge nests further away from fields. Among interior nests, such a relationship was not found. This effect was mainly brought about by avian predators (corvids) which preyed more edge nests near fields than mammalian predators.
Furthermore, the proportion of nests robbed by avian predators was high in the agricultural landscape (76.0 %, N=25), whereas mammalians were the more important predator group in the forest landscape (61.8 %, N=55).
The areas intermixed with forest patches and farmlands are expected to be more productive containing more predators than forest dominated areas (Angelstam 1986, Andren 1992). Moreover, the steepness in the productivity gradient between forest stands and surrounding open areas has been suggested to be the main factor causing elevated nest predation risk in forest edges. That is because many predators living in the surrounding matrix are expected to be capable to penetrate into forest stands or to use edges as travelling lanes (Andren & Angelstam 1988). Many of these predators have been presumed to be avians, especially corvid species (Andren et al. 1985, Andren 1992).
The results of this study confirm two of these three hypotheses. The overall predation rate did not differ between the agricultural and the forest landscapes. We did not found any edge effect on nest predation in the forest landscape but it existed in the agricultural landscape. In the northern forests clear-cuts are quite unproductive habitats in most years maybe except in vole peak years when the densities of field voles can be very high (Hansson 1979).
Thus, according to the productivity hypothesis, the edge effect may be absent in the forest landscape or exists only temporally in years when the density of voles is high in the surrounding clear-cut areas. Finally, as predicted, the proportional importance of avian predators was higher in the agricultural landscape than in the forest landscape.
3.3 Vegetation cover, nest type and visibility of nests (I, II, III)
A single branch placed above a dummy nest decreased robbing risk suggesting that nest site selection in relation to cover the habitat can provide is an important factor in nesting success of birds (study I).
In the study II predation risk was raised in nest sites with high number of pines and spruces. This effect may be related to the preference of predators over particular habitats. On the other hand, it might be also that open structure of the understory of pine forests in Lapland and spruce forests in central Finland may facilitate predators to find ground nests situated in such habitats easier.
16
In the study
m,
regardless of the edge type, significant differences in predation of nests in relation to the forest edge was not found, although the predation rate of nests were raised almost in every nest type groups. Furthermore, predation was not associated with the edge type. However, the improved visibility of nests in man-made edges due to more open vegetation structure, increased predation risk more in these habitats than in natural edges. This finding suggests that lack of vegetation cover might be an important factor exposing nests to predation. This effect, on the other hand, was not strong enough to produce differences in predation rates between the edge types. This suggests that high productivity of the later succession phase of edges may also increase predation risk possibly because predators are more attracted to such habitats.Cover and high density of vegetation have been observed to decrease nest failures of birds because heterogeneous vegetation structure conceals nests efficiently (Bowmann & Harris 1980, Yahner & Wright 1985). Clear-cutting may affect vegetation structure of the remaining forest patches increasing the width of the edge habitat. The vegetation structure of the edge zone may thus be exposed to changing microclimatical conditions and any other disturbances coming from the neighbouring open areas (Matlack 1993, Malcom 1994). This may change the vegetation structure of remaining forest patches in a way which may affect the amount of vegetation cover for breeding birds.
Nest design and a nest site both affected predation risk. The nests in the boxes suffered from lower robbing risk than the open nests. Predation of lined and unlined open nests was quite equal. However, the nests lined with hair tended to be taken more often than the control nests without lining. Thus, it might that nest design may also affect to some extent robbing risk of bird nests since some building material may increase conspicuousness of the nests for predators (study III).
3.4 Breeding success in relation to habitat structure (IV)
At macrohabitat level, pied flycatchers preferred large and medium-sized stands and avoided very small stands. The nest boxes in the small stands were mainly occupied by unpaired males which also arrived later than other males.
Reproductive success of breeding pairs was not affected by stand size.
At microhabitat level, old pied flycatcher males preferred nest boxes with high density of deciduous trees. On the contrary, yearling males occupied territories which had high numbers of pines. The territory quality was associated with the existence of these tree species since the deciduous tree dominated territories contained more invertebrate food than pine dominated ones. The reproductive output of old males was better than yearling males.
This was presumably due to the high amount of good quality invertebrate food in their territory. This age-related unequal distribution of males was likely due to male-male contest over territories in which older males dominate over younger ones.
17
The results show that pied flycatchers try to maximise their fitness by selecting breeding habitats in which their reproductive output is highest. This choice operates in different spatial scales of habitat. The net benefit of reproduction seems to be higher in stands of large size and in areas with high number of deciduous trees than in very small stands and pine dominated habitats.
Subordinate males were forced to breed in poor habitats. In these habitats the costs of reproduction presumably are higher than in good quality habitats. For example, although the overall density of invertebrates seemed to be constant across the range of stand size, the total amount of food is lower in very small stands. Thus, breeding in small forest patches may cause more costs due to increased food search distances or lower quality of food. The results are consistent with the predictions of the ideal-despotic model (Fretwell and Lucas 1970) which states that reproductive success of individuals is not equal along the quality gradient of habitats.
3.5 Edge effect and breeding success (V)
The nest boxes situated close to forest edge (50-100 m) were initially most preferred and birds avoided the nest boxes situated right at the edge (0 m).
Although the overall reproductive success did not differ between the edge and interior groups, pairs at the extreme edge produced significantly lighter fledglings than pairs in the interior areas (edges 14.2 ±0.5 g (mean±SD), N=16 and interior 14.5±0.6 g, N=20, respectively). Moreover, females of edge pairs lost more of their body mass during the nestling period (edge females 1.9±0.8 g, N=16 and interior females 1.4±0.7 g, N=19, respectively).
The avoidance of nest sites near extreme edge area seems to be due to lower quality of these sites for breeding. Nest site choice was, however, not related to lower amount of food supply in the forest edge than in the interior area since the abundance and quality of food did not differ between the areas.
Instead, it seems plausible that differences in parental care between the study groups may explain the results observed. However, because quality of parents was also controlled, it is probable that mass reduction of broods and females might be a result from environmental factors affecting the parental care.
According to the central place foraging theory (e.g. Andersson 1978), the optimal foraging area should be round. However, at the extreme edge the foraging area of pied flycatchers is a half circle in shape because they did not forage on open areas. Edge pairs are thus forced to increase the radius of their foraging area to obtain the same sized forested area as in the interior part of a stand which ,in turn, increases their travelling distances between foraging and nest sites. The increased travelling distance can be very costly for parents during nestling period when parents feed the nestling about every other minute (Lundberg & Alatalo 1992).
Thus, the direct fitness costs caused by breeding at the extreme edge would be an increased energy consumption and possibly increased predation risk as a result of longer flying distances. The indirect fitness costs would be
18
the lower quality of fledglings produced and possibly the lower survival rate and the lower reproductive value of parents in future.